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Christmas crabs

Klaus Esterluss
December 21, 2017

What happens if you give decorator crabs access to fluffy pom-poms? The animals will look like Christmas. But there is method to their seasonal attire.  

 A decorator crab dressed in green and red pom poms
Image: Rohan Brooker

Christmas is known as the season in which oh-so-many people appear to develop a strange longing to dress in colorful patterned jumpers that wouldn't stand a chance throughout the rest of the year. It is athing. 

Camposcia retusa or decorator crabs, as they are more commonly known, do things differently. Though, as their name suggests, they enjoy dressing up - using objects from their environment - they at least have a reason for it.

Fluffy little balls

This desire to adorn themselves, makes the crabs much loved among marine scientists. Usually they would choose to dress up in sponges, algae and marine debris, but as Danielle Dixson and her team from the University of Delaware found out during recent research, they don't shy away from pom-poms that give them a more festive look.

The team of scientists, which included undergraduate students, ran a series of experiments in which crabs were placed in individual containers and provided with green and red Christmas-style pom-poms.

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"We chose to use craft pom poms because they were light enough that the crab would be able to use them and we can easily manipulate them in terms of color and size," says Dixson. The colors used in the study were simply based on the pom-poms that were available at a store in multiple sizes, she adds.

The make-up

Half the crabs were given shelters. Dixson wanted to know if there was any difference between the speed and manner of decoration if the crabs had a place to hide. During the course of a full day, the team took photos every 60 minutes for the first 12 hours, and again at the end of the 24 hour period.

They found that all of the crabs were fully decorated within 24 hours. Most were in fact decorated within six hours of having access to the pom-poms.

"This indicates the importance of the decorating behavior", Dixson says. "They also decorate their legs before they decorate their body."

Species that use decoration usually cover the body first to protect their vital organs, like the larvae of many caddisfly species that build hard cases from whatever material they find, or assassin bugs who have the odd habit of carrying a shield of ant carcasses - their number one prey - to avoid being eaten by predators.

The crabs that were given shelter were in a hurry to first decorate whichever legs did not fit inside - because they were more exposed.

In the end, both humans and crabs share a certain danger by decoration: The crustaceans, when all dressed-up, require more energy to move around, and eventually become too slow to escape predators, while humans are at risk to be bullied by Christmas humbugs for wearing something really ugly. Happy holidays.